1. Field of the Invention
The present invention relates to a semiconductor laser element including an ARROW (Antiresonant Reflecting Optical Waveguide) structure. In particular, the present invention relates to a semiconductor laser element including an ARROW structure and emitting laser light in the 980 nm band.
2. Description of the Related Art
A reliable high-power semiconductor laser element which emits a high-quality, diffraction-limited beam is required for use as a light source in exciting an optical fiber amplifier.
U.S. Pat. No. 5,606,570 discloses a semiconductor laser element having an ARROW structure as a semiconductor laser element which can emit a high-output-power, diffraction-limited laser beam in the 980 nm band. The disclosed semiconductor laser element includes an InGaAs active layer and an InGaAlP current confinement layer, and uses GaAs as a medium having a high refractive index. The ARROW structure is a structure for confining light in core regions. The disclosed ARROW structure includes a plurality of core regions having a low equivalent (effective) refractive index, first high-refractive-index regions which have a high equivalent refractive index and are arranged between the plurality of core regions and on the outer sides of the plurality of core regions, low-refractive-index regions which have an equivalent refractive index approximately identical to that of the plurality of core regions and are arranged on the outer sides of the outermost ones of the high-refractive-index regions, and second high-refractive-index regions which have a high equivalent refractive index and are arranged on the outer sides of the low-refractive-index regions. The first high-refractive-index regions behave as reflectors of light in the fundamental transverse mode, and the low-refractive-index regions suppress leakage of light. Thus, the semiconductor laser element can be controlled so as to operate in the fundamental transverse mode.
It is reported that a preferable value of the width db1xe2x80x2 of each of the outermost ones of the first high-refractive-index regions is determined in accordance with the equation (1), a preferable value of the width db2xe2x80x2 of each of the first high-refractive-index regions arranged between the plurality of core regions is determined in accordance with the equation (2), and a preferable value of the width of each of the low-refractive-index regions is dc/2, where dcxe2x80x2 is the width of each of the plurality of core regions. In the equations (1) and (2), xcex is the oscillation wavelength, ncxe2x80x2 is the equivalent refractive index of the plurality of core regions, and nbxe2x80x2 is the equivalent refractive index of the first high-refractive-index regions.                               d          b1          xe2x80x2                =                                            (                                                2                  ⁢                  m                                +                1                            )                        ⁢            λ                                4            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        1        )                                          d          b2          xe2x80x2                =                              m            ⁢                          xe2x80x83                        ⁢            λ                                2            ⁢                                          {                                                      n                    b                    xe2x80x22                                    -                                      n                    c                    xe2x80x22                                    +                                                            (                                              λ                                                  2                          ⁢                                                      d                            c                            xe2x80x2                                                                                              )                                        2                                                  }                                            1                2                                                                        (        2        )            
The semiconductor laser elements disclosed in U.S. Pat. No. 5,606,570 have a structure which requires a regrowth technique. According to the structure, InGaP, InAlP, or GaAs layers are exposed at the surface as a base of the regrowth at the time of the regrowth. Therefore, P-As interdiffusion occurs at the exposed surface during a process of raising temperature for the regrowth, and thus the regrowth is likely to become defective. As a result, the above semiconductor laser element is not practicable.
An object of the present invention is to provide a semiconductor laser element which includes an ARROW structure and is reliable in a wide output power range from low to high output power levels.
Another object of the present invention is to provide a process for producing a semiconductor laser element which includes an ARROW structure and is reliable in a wide output power range from low to high output power levels.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser element comprising: a GaAs substrate of a first conductive type; a lower cladding layer formed above the GaAs substrate and made of In0.49Ga0.51P or Alz1Ga1-z1As of the first conductive type, where 0.2xe2x89xa6z1xe2x89xa60.8; a lower optical waveguide layer formed above the lower cladding layer and made of GaAs which is undoped or the first conductive type; a compressive-strain quantum-well active layer formed above the lower optical waveguide layer and made of undoped Inx3Ga1-x3As1-y3Py3 where 0.49y3xe2x89xa6x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; an upper optical waveguide layer formed above the compressive-strain quantum-well active layer and made of GaAs which is undoped or a second conductive type; a first etching stop layer made of Inx8Ga1-x8P of the second conductive type and formed above the upper optical waveguide layer other than stripe areas of the upper optical waveguide layer corresponding to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, where 0xe2x89xa6x8xe2x89xa61, and the stripe areas extend in an oscillation direction of a laser resonator; a first current confinement layer made of GaAs of the first conductive type and formed above the first etching stop layer; a second etching stop layer made of Inx9Ga1-x9P of the first conductive type or the second conductive type and formed above the first current confinement layer and ones of the stripe areas of the upper optical waveguide layer corresponding to the low-refractive-index regions, where 0xe2x89xa6x9xe2x89xa61; a second current confinement layer made of Alz1Ga1-z1As of the first conductive type and formed above the second etching stop layer; an upper cladding layer made of Alz1Ga1-z1As of the second conductive type and formed above the second current confinement layer and at least one of the stripe areas of the upper optical waveguide layer corresponding to the at least one current injection region; and a contact layer made of GaAs of the second conductive type and formed above the upper cladding layer.
Preferably, the semiconductor laser element according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The semiconductor laser element according to the first aspect of the present invention may further comprise a cap layer made of GaAs of the first conductive type or the second conductive type and formed on the second current confinement layer made of Alz1Ga1-z1As of the first conductive type.
(ii) The semiconductor laser element according to the first aspect of the present invention may further comprise a GaAs layer of the second conductive type and formed between the first etching stop layer and the first current confinement layer, where the first etching stop layer is made of Inx8Ga1-x8P of the second conductive type, and the first current confinement layer is made of GaAs of the first conductive type.
(iii) The semiconductor laser element according to the first aspect of the present invention may further comprise an InGaAs quantum-well layer formed at a mid-thickness of the first current confinement layer, where the InGaAs quantum-well layer has a bandgap smaller than the bandgap of the active layer.
(iv) It is preferable that the width of each of the at least one current injection region is 3 micrometers or greater.
(2) According to the second aspect of the present invention, there is provided a process for producing a semiconductor laser element, comprising the steps of: (a) forming above a GaAs substrate of a first conductive type a lower cladding layer made of In0.49Ga0.51P or Alz1Ga1-z1As of the first conductive type, where 0.2xe2x89xa6z1xe2x89xa60.8; (b) forming above the lower cladding layer a lower optical waveguide layer made of GaAs which is undoped or the first conductive type; (c) forming above the lower optical waveguide layer a compressive-strain quantum-well active layer made of undoped Inx3Ga1-x3As1-y3Py3, where 0.49y3 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; (d) forming above the compressive-strain quantum-well active layer an upper optical waveguide layer made of GaAs which is undoped or a second conductive type; (e) forming above the upper optical waveguide layer a first etching stop layer made of Inx8Ga1-x8P of the second conductive type, where 0xe2x89xa6x8xe2x89xa61; (f) forming above the first etching stop layer a first current confinement layer made of GaAs of the first conductive type; (g) removing stripe regions of the first current confinement layer and the first etching stop layer so as to expose first stripe areas of the upper optical waveguide layer, where the first stripe areas correspond to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, and the first stripe areas extend in an oscillation direction of a laser resonator; (h) raising temperature in an arsenic atmosphere; (i) forming a second etching stop layer made of Inx9Ga1-x9P of the first conductive type or the second conductive type by regrowth above remaining areas of the first current confinement layer and the first stripe areas of the upper optical waveguide layer, where 0xe2x89xa6x9xe2x89xa61; (j) forming above the second etching stop layer a second current confinement layer made of Alz1Ga1-z1As of the first conductive type; (k) removing stripe regions of the second current confinement layer and the second etching stop layer so as to expose at least one second stripe area of the upper optical waveguide layer, where the at least one second stripe area corresponds to the at least one current injection region, and the at least one second stripe area extends in the oscillation direction of the laser resonator; (l) raising the temperature in an arsenic atmosphere; (m) forming an upper cladding layer made of Alz1Ga1-z1As of the second conductive type by regrowth above remaining areas of the second current confinement layer and the at least one second stripe area of the upper optical waveguide layer; and (n) forming above the upper cladding layer a contact layer made of GaAs of the second conductive type.
Thus, the semiconductor laser element according to the first aspect of the present invention can be produced by the process according to the second aspect of the present invention.
A layer made of GaAs of the first conductive type or the second conductive type may be formed on the second current confinement layer.
(3) According to the third aspect of the present invention, there is provided a semiconductor laser element comprising: a GaAs substrate of a first conductive type; a lower cladding layer formed above the GaAs substrate and made of Alz1Ga1-z1As or In0.49(Alz2Ga1-z2)0.51P of the first conductive type, where 0.57xe2x89xa6z1xe2x89xa60.8 and 0.1xe2x89xa6z2xe2x89xa61; a lower optical waveguide layer formed above the lower cladding layer and made of In0.49Ga0.51P which is undoped or the first conductive type; a compressive-strain quantum-well active layer formed above the lower optical waveguide layer and made of undoped Inx3Ga1-x3As1-y3Py3, where 0xe2x89xa6x3xe2x89xa60.4 and 0xe2x89xa6y3 xe2x89xa60.5; an upper optical waveguide layer formed above the compressive-strain quantum-well active layer and made of In0.49Ga0.51P which is undoped or a second conductive type; a first etching stop layer made of Inx4Ga1-x4As1-y4Py4 of the first conductive type or the second conductive type and formed above the upper optical waveguide layer other than stripe areas of the upper optical waveguide layer corresponding to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, where 0xe2x89xa6x4xe2x89xa61 and 0xe2x89xa6y4xe2x89xa60.8, and the stripe areas extend in an oscillation direction of a laser resonator; a first current confinement layer made of In0.49Ga0.51P of the first conductive type and formed above the first etching stop layer; a second etching stop layer made of Inx4Ga1-x4As1-y4Py4 of the first conductive type and formed above the first current confinement layer and ones of the stripe areas of the upper optical waveguide layer corresponding to the low-refractive-index regions, where 0xe2x89xa6x4xe2x89xa61 and 0xe2x89xa6y4xe2x89xa60.8; a second current confinement layer made of In0.49(Alz2Ga1-z2)0.51P of the first conductive type and formed above the second etching stop layer, where 0.1xe2x89xa6z2xe2x89xa61; an upper cladding layer made of In0.49(Alz2Ga1-z2)0.51P of the second conductive type or AlGaAs which has a refractive index approximately identical to a refractive index of the second current confinement layer, and formed above the second current confinement layer and at least one of the stripe areas of the upper optical waveguide layer corresponding to the at least one current injection region; and a contact layer made of GaAs of the second conductive type and formed above the upper cladding layer.
Preferably, the semiconductor laser element according to the third aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iii).
(i) The semiconductor laser element according to the third aspect of the present invention may further comprise a cap layer made of In0.49Ga0.51P of the first conductive type or the second conductive type and formed on the second current confinement layer made of In0.49(Alz2Ga1-z2)0.51P of the first conductive type.
(ii) It is preferable that the width of each of the at least one current injection region is 3 micrometers or greater.
(iii) The semiconductor laser element according to the third aspect of the present invention may further comprise an InGaAsP quantum-well layer formed at a mid-thickness of the first current confinement layer, where the InGaAsP quantum-well layer has a bandgap smaller than the bandgap of the active layer.
(4) According to the fourth aspect of the present invention, there is provided a process for producing a semiconductor laser element, comprising the steps of: (a) forming above a GaAs substrate of a first conductive type a lower cladding layer made of Alz1Ga1-z1As or In0.49(Alz2Ga1-z2)0.51P of the first conductive type, where 0.57xe2x89xa6z1xe2x89xa60.8 and 0.1xe2x89xa6z2xe2x89xa61; (b) forming above the lower cladding layer a lower optical waveguide layer made of In0.49Ga0.51P which is undoped or the first conductive type; (c) forming above the lower optical waveguide layer a compressive-strain quantum-well active layer made of undoped Inx3Ga1-x3As1-y3Py3, where 0xe2x89xa6x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.5; (d) forming above the compressive-strain quantum-well active layer an upper optical waveguide layer made of In0.49Ga0.51P which is undoped or a second conductive type; (e) forming above the upper optical waveguide layer a first etching stop layer made of Inx4Ga1-x4As1-y4Py4 of the first conductive type or the second conductive type, where 0xe2x89xa6x4xe2x89xa61 and 0xe2x89xa6y4xe2x89xa60.8; (f) forming above the first etching stop layer a first current confinement layer made of In0.49Ga0.51P of the first conductive type; (g) removing stripe regions of the first current confinement layer and the first etching stop layer so as to expose first stripe areas of the upper optical waveguide layer, where the first stripe areas correspond to at least one current injection region and low-refractive-index regions located on outer sides of the at least one current injection region and separated from the at least one current injection region or outermost ones of the at least one current injection region by a predetermined interval, and the first stripe areas extend in an oscillation direction of a laser resonator; (h) raising temperature in a phosphorus atmosphere; (i) forming a second etching stop layer made of Inx4Ga1-x4As1-y4Py4 of the first conductive type by regrowth above remaining areas of the first current confinement layer and the first stripe areas of the upper optical waveguide layer; (j) forming above the second etching stop layer a second current confinement layer made of In0.49(Alz2Ga1-z2)0.51P of the first conductive type; (k) removing stripe regions of the second current confinement layer and the second etching stop layer so as to expose at least one second stripe area of the upper optical waveguide layer, where the at least one second stripe area corresponds to the at least one current injection region, and the at least one second stripe area extends in the oscillation direction of the laser resonator; (l) raising the temperature in a phosphorus atmosphere; (m) forming an upper cladding layer of the second conductive type made of In0.49(Alz2Ga1-z2)0.51P or AlGaAs which has a refractive index approximately identical to a refractive index of the second current confinement layer, by regrowth above remaining areas of the second current confinement layer and the at least one second stripe area of the upper optical waveguide layer; and (n) forming above the upper cladding layer a contact layer made of GaAs of the second conductive type.
Thus, the semiconductor laser element according to the third aspect of the present invention can be produced by the process according to the fourth aspect of the present invention.
In the above descriptions of the first to fourth aspects of present invention, the first conductive type is different in the polarity of carriers from the second conductive type. That is, when the first conductive type is n type, the second conductive type is p type.
In addition, the term xe2x80x9cundopedxe2x80x9d means that a material is not doped with any conductive impurity.
(5) The advantages of the present invention are as follows.
(i) In the semiconductor laser elements according to the first and second aspects of the present invention, the following structure is formed in the direction perpendicular to the thickness direction and the light propagation direction in the active layer. That is, first high-refractive-index regions which have a relatively high equivalent refractive index are realized between at least one core region (corresponding to the at least one current injection region each of which has a stripe shape) and on the outer sides of the at least one core region, low-refractive-index regions which have a relatively low equivalent refractive index are realized on the outer sides of the outermost ones of the first high-refractive-index regions, and second high-refractive-index regions which have a relatively high equivalent refractive index are realized on the outer sides of the low-refractive-index regions. That is, the aforementioned ARROW structure is realized.
Since the semiconductor laser elements according to the first and second aspects of the present invention include an ARROW structure, the semiconductor laser element according to the present invention can emit a single peak beam in a transverse mode which is more effectively controlled than that in semiconductor laser elements which do not include the ARROW structure, even when the stripe width is increased.
In order to effectively control the transverse mode oscillation in the semiconductor laser elements which do not include the ARROW structure, the stripe width is required to be reduced to 3 micrometers or smaller, i.e., the width of the active region is required to be reduced. Therefore, when the output power is increased, the optical density in the active layer increases, and thus facet degradation is likely to occur. Consequently, the semiconductor laser elements which do not include the ARROW structure cannot operate with high output power in an effectively controlled transverse mode.
On the other hand, since the semiconductor laser elements according to the first and second aspects of the present invention include the ARROW structure, light can be satisfactorily confined in a wide stripe (active) region, and therefore the semiconductor laser element according to the present invention can emit laser light in the fundamental transverse mode from the wide active region.
(ii) In particular, when the width of the active region is increased to 3 micrometers or greater, the optical density in the active layer can be reduced, and therefore the temperature rise due to non-radiative recombination in vicinities of end facets can be decreased. Thus, the semiconductor laser elements according to the first and second aspects of the present invention can emit a laser beam in the fundamental transverse mode with higher power than the semiconductor laser elements which do not include the ARROW structure.
(iii) In the semiconductor laser element according to the first aspect of the present invention, the optical waveguide layers and the first current confinement layer are made of GaAs. Therefore, refractive indexes of the high-refractive-index regions can be easily controlled. That is, it is possible to obtain an ARROW structure in which the distribution of the refractive index in the horizontal directions can be easily controlled.
(iv) When an InGaAs quantum-well layer is formed at a mid-thickness of the first current confinement layer in the semiconductor laser element according to the first aspect of the present invention, and the InGaAs quantum-well layer has a bandgap smaller than the bandgap of the active layer, the gain in the oscillation in the fundamental transverse mode can be increased since the InGaAs quantum-well layer absorbs light.
(v) When the process according to the second aspect of the present invention is used, it is possible to easily produce a semiconductor laser element having an ARROW structure with high precision.
(vi) In the semiconductor laser element according to the first aspect of the present invention, almost only the AlGaAs current confinement layer and the GaAs upper optical waveguide layer are exposed at the surface as a base of the regrowth. That is, As and P do not coexist on the surface which is exposed at the time of an operation of regrowth. Therefore, P-As interdiffusion does not occur, and thus the quality of the regrown crystal can be improved.
(vii) In the semiconductor laser element according to the first aspect of the present invention, the etching stop layers are made of InGaP. Therefore, the accuracy of the etching of the GaAs layers is increased, and it is possible to precisely realize a distribution of the equivalent refractive index which is necessary for an ARROW structure.
(viii) When a cap layer made of GaAs of the first conductive type or the second conductive type is formed on the second current confinement layer (made of Alz1Ga1-z1As of the first conductive type) in the semiconductor laser element according to the first aspect of the present invention, only small portions of the AlGaAs second current confinement layer are exposed at the time of the regrowth of the upper cladding layer, and therefore aluminum oxidation can be reduced. Thus, it is possible to improve the crystal quality and the reliability.
(ix) Since the semiconductor laser element according to the second aspect of the present invention includes the optical waveguide layers made of In0.49Ga0.51P and the first current confinement layer made of In0.49Ga0.51P of the first conductive type, the distribution of the equivalent refractive index for the ARROW structure can be easily realized by controlling the thickness of the In0.49Ga0.51P upper optical waveguide layer. In addition, the refractive index of In0.49Ga0.51P, of which the first current confinement layer is made, is higher than that of InGaAlP, and the first current confinement layer can also behave as an additional portion of the upper optical waveguide layer. Therefore, the equivalent refractive index of the high-refractive-index regions can be easily controlled, and the light can be efficiently confined. Thus, it is possible to obtain high-quality laser light.
(x) In the semiconductor laser element according to the second aspect of the present invention, almost only the InGaP first current confinement layer is exposed at the surface as a base of the regrowth of the second etching stop layer. Since As and P do not coexist on the surface, P-As interdiffusion does not occur. Therefore, it is possible to improve the quality of the regrown crystal, and obtain a reliable semiconductor laser element.
(xi) In the semiconductor laser element according to the second aspect of the present invention, the first and second etching stop layers are made of InGaAsP, and the layer adjacent to the first and second etching stop layers is made of InGaP. Therefore, it is possible to increase the accuracy of the etching, and precisely realize the distribution of the equivalent refractive index which is necessary for the ARROW structure.
(xii) When a cap layer made of In0.49Ga0.51P of the first conductive type or the second conductive type is formed on the second current confinement layer (made of In0.49(Alz2Ga1-z2)0.51P of the first conductive type) in the semiconductor laser element according to the second aspect of the present invention, only small portions of the In0.49(Alz2Ga1-z2)0.51P second current confinement layer are exposed at the time of the regrowth of the upper cladding layer. Therefore, degradation caused by aluminum oxidation can be prevented. Thus, it is possible to improve the crystal quality and the reliability.
(xiii) When an InGaAsP quantum-well layer is formed at a mid-thickness of the first current confinement layer in the semiconductor laser element according to the second aspect of the present invention, and the InGaAsP quantum-well layer has a bandgap smaller than the bandgap of the active layer, the gain in the oscillation in the fundamental transverse mode can be increased since the InGaAsP quantum-well layer absorbs light.
(xiv) When the process according to the second aspect of the present invention is used, it is possible to easily produce a semiconductor laser element having an ARROW structure with high precision. In addition, since the temperature is raised in a phosphorus atmosphere, the regrowth becomes easy, and the crystal quality of the regrown layers such as the InGaAlP layer can be improved.